Many structures, such as vehicles, vibrate during operation. Further, each structure has a natural or resonant frequency that depends upon its shape or configuration. Structures of various types are frequently designed so that operational vibration does not match the natural frequency of the structure, so as to avoid natural or resonant frequencies. This is done to prevent damage to the structure, since a resonant frequency can focus mechanical energy into a specific location or part of the structure, and exceed the material strength of the structure at that location, potentially causing damage or failure.
One category of vehicles that are affected by this issue is aircraft, such as rotary wing aircraft (e.g. helicopters). Rotary wing aircraft are frequently subject to high vibration environments. Levels of vibration vary based on factors such as the rotor speed(s), environmental factors, and payload. Vibration levels, in turn, often determine or limit the size and weight of a given rotary wing aircraft, or limit rotor speeds during its operation. To deal with these issues, existing rotorcraft airframes are frequently stiffened by adding mass to the structure in order to avoid undesirable (e.g. resonant) frequencies of vibration. In the case of rotorcraft, the added mass can be significant—perhaps hundreds of pounds. Improved load-bearing materials can be helpful in addressing this issue, but generally do not, on their own, allow for a reduction in vibration tuning mass. Furthermore, addition of parasitic weight can restrict the speed and payload capabilities of the aircraft, yet it is generally considered more desirable to have a lighter weight aircraft to allow more payload and/or higher fuel efficiency.
In addition to the selective addition of weight, another approach for avoiding natural or resonant frequencies is to only operate a vehicle within certain operational ranges, so that only certain vibrational frequencies are possible. However, this approach limits the utility of the vehicle.
The present application relates to one or more of the above issues.